JavaScript and regular expressions: literal syntax Vs. RegExp object

I have some troubles with this small JavaScript code:

var text="Z Test Yeah ! Z";

// With literal syntax, it returns true: good!
alert(/(Z[\s\S]*?Z)/g.test(text));

// But not with the RegExp object O_o
var reg=new RegExp('Z[\s\S]*?Z','g');
alert(reg.test(text));

I don’t understand why the literal syntax and the RegExp object don’t give me the same result…
The problem is that I have to use the RegExp object since I’ll have some variables later.

Any ideas?

Thanks in advance 🙂